The present invention relates to the drilling of drainholes in subsurface formations and more particularly to the use of a measurement while drilling device to allow correction of drainhole direction to avoid crossing boundaries of a selected subsurface formation.
The drilling of lateral bores from a main vertical borehole is a well developed art. For example, U.S. Pat. No. 2,336,338 issued to Zublin on Dec. 7, 1943 and U.S. Pat. No. 3,398,804 issued to Holbert on Aug. 27, 1968 each teach various equipment and methods for drilling a curved bore away from an existing vertical borehole. Both of these patents are incorporated by reference for all purposes. The Zublin patent teaches the use of a pre-curved guide to begin the curved portion of the horizontal bore. After a sufficient bore angle has been achieved, Zublin teaches the use of a straight guide to continue the horizontal bore in a straight direction away from the main vertical bore.
In practice, considerable problems have been encountered in attempting to control the direction of lateral or horizontal bores extending from vertical wells. Such problems have often limited the extent of the horizontal bore to several hundred feet from the vertical borehole. In many cases it would be desirable to extent such lateral bores to 1000 feet or more from the vertical bore.
The use of well survey tools is a well-known technique for determining the actual locations of a bore. However this technique is time consuming and expensive since it usually requires that the drill string be pulled from the borehole during the surveying process. When it is determined that the horizontal bore is not proceeding in the desired direction, techniques are know for curving the bore to correct for the errors. For example, a mud driven hydraulic motor may be used on the drill string to drive the drill bit. This motor and bit combination can easily be arranged to drill in a continuous curve. When a straight hole is desired the entire drill string may be slowly rotated during the drilling operation to counteract the tendency of the bit to drill the curved hole. When correction is needed, the drill string may be oriented appropriately so that the motor and bit are allowed to drill a curved section of hole to compensate for directional errors detected by a survey device.
In addition to the time and expense involved in the process of repeating surveys and corrections, another problem is encountered with such a process especially where very long lateral holes are desired. The precise upper and lower boundaries of subsurface formations are often not precisely known especially at any reasonable distance from the borehole. Hydrocarbon bearing formations are normally neither perfectly horizontal nor perfectly flat. Thus the upper and lower boundaries of the formation will generally slant or dip with respect to true horizontal and will have localized nonuniformities. As a result, even if the well surveying techniques were perfect, it would not be possible to determine precisely where the lateral bore should be placed to keep it within the producing zone of interest.